But she heard it again and again. Her neighbors started hearing it. Police were called. They received 114 calls about the noises since mid-February, according to Chris Paulson, the city's administrative services director. He told the Pasadena Star-News local government agencies did not know what the cause of the sound was.

City officials are baffled; one blast interrupted a city council meeting.

"All of a sudden we hear this loud sharp explosion — very quick," Paulson said. "We all flinched and looked around and didn’t see smoke or flames or light."

"There's nothing seismic that I can see," said Jennifer Andrews, a staff seismologist at Caltech in Pasadena, who was asked by Alhambra city officials to check earthquake data for Feb. 22. "What that phenomenon might be, I don't know. I haven't heard the noises."

The seismograph picks up pressure waves from things like thunder and helicopter sounds. But the Alhambra explosions are baffling.